Prepress color proofing systems are in widespread use in the printing industry. In a typical process, a multicolor original is separated into individual transparencies, called color separations, for the three subtractive primaries and for black. The separation process can be carried out in a number of well-known ways. For example, a graphic arts scanner can be used to create the color separations. In some instances, more than four color separations are employed. A color proof, called an "off-press" proof or a "prepress" proof, is then prepared from the color separations. The color proof is used by the printer to check color balance and other important quality control parameters.
Generally speaking, prepress color proofs are one of three types: namely (1) a color overlay that employs an image on a separate base for each color, (2) a single integral sheet process in which the separate color images are combined on a single base, or (3) a digital method in which the images are produced directly on a single base from digital data. The overlay and single sheet process are described in some detail in, for example, U.S. Pat. No. 4,895,787 (Platzer) which points out that the single sheet process is greatly preferred because the superposed supports of the overlay process drastically alter the appearance of the color proof.
U.S. Pat. No. 3,622,320 (Allen) describes a color proofing process of the single sheet type that is simple and convenient to use, and is a dry process that does not require the use of processing solutions. As described therein, the color-proofing process utilizes two or more light-sensitive donor elements, each of which contains a different dispersed colorant. The donor elements employ a light-sensitive resin that has a tackifying point that is raised by light exposure. That is, the resin is photohardenable. The imagewise-exposed donor element is pressed into contact with a receiver element while being heated to a temperature that is greater than the tackifying point in unexposed areas of the resin but less than the tackifying point in the exposed areas of the resin. Thus, the colored resin transfers from the unexposed areas to the receiver element. Second and subsequent donor elements, each containing a different colorant, are similarly exposed, and the image is transferred therefrom, in exact registration, to the same receiver element. For full color reproduction, it is customary to expose and transfer images from donor elements containing, respectively, black, magenta, cyan and yellow colorants. Commonly, the donor elements are exposed by use of a set of half-tone color separation positives.
Instead of using a photohardenable resin, acceptable results in a single sheet process can also be achieved by using a donor resin that is phototackifiable. With this type of resin, the exposed areas will be more tacky at the transfer temperature than the unexposed areas and, accordingly, the colored resin will transfer from the exposed areas to the receiver element. In this instance, the donor elements can be exposed by use of a set of half-tone color separation negatives. Phototackifiable compositions and their use in a single sheet process are described, for example, in U.S. Pat. No. 5,108,868 (Platzer).
Whatever donor resin is used, the process of making a color proof requires heat to increase the difference in surface adhesion between exposed and unexposed areas of the donor element and pressure to bring about effective image transfer. For example, as described in U.S. Pat. No. 5,374,497 (Kapusniak et al), the exposed donor element is placed in contact with the receiver element, the laminate that is so formed is passed between a pair of heated pressure rollers, and the donor element is then separated from the receiver element which then carries the transferred image. Separation of the donor element from the receiver element is usually accomplished by a manual peeling step, but can, if desired, be accomplished automatically by the laminator.
The noted laminate can be carried through the rollers using a dimensionally stable carrier plate that may be merely a metal sheet, or that employs a dimensionally stable receiver.
After the first image is transferred, it is critical that there be exact registration of each of the subsequently transferred images. One common way of accurately registering the successively transferred images is by means of a pin registration system.
A more advantageous means for registration is described in U.S. Pat. No. 5,374,497 (noted above). This patent describes dry color proofing processes wherein a colored image is transferred by the steps of hot lamination and peeling of successive donor elements and a single "intermediate" receiver element. The multicolor image can then be subsequently transferred from the intermediate receiver element to a final receiver element. Building the color image on the intermediate receiver element is advantageous because it permits the formation of an image that is not laterally reversed.
The noted patent describes the intermediate receiver element as being composed of any of a wide variety of pollEneric films such as polyesters, polyvinyl acetates, polyacrylates, polymethacrylates and polystyrenes. Optionally, the film base can be coated with a thin layer, such as a vinyl acetate, that transfers along with the multicolor image.
In a hot peel system, the donor element polymers that are transferred to the intermediate receiver element are somewhat fluid because the hot peeling step takes place at temperatures above the glass transition temperature of the donor element polymer. As a result, the donor element polymers tend to flow, in a non-imagewise manner, in the direction of peel during the hot peeling step. The result of such flow is a loss in image resolution.
Resolution, for example, can be expressed as the largest "% dot" resolvable on the intermediate receiver element when exposed to a 50 lines/cm screen. A high quality color proofing system would be able to resolve a 98% dot with a 50 line/cm screen. A 98% dot resolution (with a 50 line/cm screen) means that the image transferred to the intermediate receiver element should have 50 "holes" (areas of no colorant) per centimeter, and the "holes" comprise 2% of the total image area.
An intermediate receiver element of the prior art, which has a smooth outer surface, will not have the required 50 "holes"/cm because the donor polymers will flow into the "holes" during the hot peeling step. As a result, the colorant will cover the entire image area and the dots will not be resolvable using the noted screen.
In addition to the problem of poor resolution caused by flow during the hot peeling step, a further problem exists that has hindered the successful commercialization of the known dry color proofing system described in U.S. Pat. No. 5,374,497 (noted above).
During the lamination step, each donor element and the intermediate receiver element are brought into intimate contact to form a laminate. Any foreign materials, referred to herein as "dirt", that are trapped between a donor element and the intermediate receiver element can interfere with the required intimate contact. The "dirt" particle, if large enough, can cause what is known as a "minus density spot defect". In other words, the dirt particle may cause an absence of colorant in a particular region of the image, and the diameter of this region is proportional to the height of the dirt particle.
The intermediate receiver elements described in the Kapusniak et al patent (noted above) suffer from both of these problems, that is inadequate resolution of the image because of flow of the transferred donor polymer, and an unacceptable number of minus density spot defects caused by entrapped dirt particles.
Still another problem associated with known dry color proofing processes relates to the carrier plates used to transport the donor and intermediate receiver elements through the laminating rollers. In the lamination step, at least one of the elements being laminated must be heated sufficiently to deliver suitable heat to the interface of the two elements. If the lamination temperature is above the glass transition temperature of the intermediate receiver or donor element support, an irreversible dimensional change will occur due to the relaxation of residual stresses within either element's support. Simple carrier plates known in the art, such as those described in U.S. Pat. No. 5,374,497 (noted above), are not able to prevent this highly undesirable irreversible dimensional change.
Thus, there is a considerable need for a method that provides a dry prepress color proof using intermediate receiver elements and carrier plates that do not suffer from the problems noted above. Such a method should be adaptable to existing equipment, reliable and simple to use.